The present invention relates to a pressurized-gas storage battery, and, more particularly, to such a battery structured for high efficiency and long-term stability.
Rechargeable cells or batteries are electrochemical devices for storing and retaining an electrical charge and later delivering that charge for useful power. Familiar examples of the rechargeable cell are the lead-acid cell used in automobiles, and the nickel-cadmium cell used in various portable electronic devices. Another type of cell having a greater storage capacity for its weight is the nickel oxide/pressurized hydrogen cell, an important type of which is commonly called the nickel-hydrogen cell and is used in spacecraft applications. The weight of the spacecraft storage cell must be minimized while achieving the required performance level, due to the cost of lifting weight to earth orbit and beyond.
The nickel-hydrogen cell includes a series of active plate sets which store an electrical charge electrochemically and later deliver that charge as a useful current. The active plate sets are packaged within a pressure vessel that contains the plate sets and the hydrogen gas that is an essential active component of the cell. Each plate set includes a positive electrode, a negative electrode, and a separator between the two electrodes, all soaked with an electrolyte. In a typical cell, a number of plate sets are supported on a core under a compressive loading, with a gas screen between each plate set and with electrical connector leads extending to each electrode of each plate set. The gas screen provides a gas channel from the hydrogen electrode to the gas space outside the stack. A single nickel-hydrogen storage cell delivers current at about 1.3 volts, and a number of the cells are usually connected in series to produce current at the voltage required by the systems of the spacecraft.
Although the nickel-hydrogen battery cell has higher theoretical specific energy (electrical energy storage capacity per unit weight) than the nickel-cadmium battery cell, it is bulky and has lower energy density (electrical energy storage capacity per unit volume) than the nickel-cadmium battery cell. The cylindrical shape of the pressure vessel of the nickel-hydrogen battery cell, as compared with the rectilinear shape of the nickel-cadmium battery cell, adds a significant volume of unusable space to the battery. The pressure vessel cannot be significantly redesigned to improve the volumetric efficiency properties of the nickel-hydrogen battery, because its shape is optimized for maximum weight properties while confining the hydrogen pressure that can reach as high as 1000 pounds per square inch during the battery charge/discharge cycle.
There is therefore a need for an improved nickel-hydrogen battery which retains its good weight properties but achieves improved volumetric properties. The battery must also retain its operating characteristics for a long period of time, typically on the order of 15 years for a spacecraft such as a commercial communications satellite. That is, there must be a good certainty that any modification to the nickel-hydrogen battery will not cause the battery to change its performance during the design life. Since it is not possible to conduct 15 year full-life tests of the battery prior to the launch of the satellite, possible failure mechanisms must be anticipated and avoided in the basic battery design.
The present invention fulfills this need for an improved pressurized-gas battery having good weight and volumetric properties, and no added long-term failure mechanisms.